The invention relates to a glass for a cone of a display tube.
The invention also relates to a cone and a display tube envelope whose cone is made from such a glass.
The invention further relates to a method of manufacturing cone glass and to a method of manufacturing a cone from such a glass.
Display tubes, such as display tubes for TV receivers and computer monitors, have an evacuated glass envelope which is built up of a screen, a cone and a neck. The neck is the part of the envelope which contains one or more electron guns. The screen contains, inter alia, one or more luminescent substances (phosphors). The cone is the funnel-shaped portion of the display tube. These three parts have different glass compositions whose physical properties are adapted to each other. Examples of important physical properties are: melting temperature, thermal expansion coefficient, x-ray absorption and electrical resistivity. The screen and the cone are manufactured by pressing a molten glass mixture and are sealed together by means of a sealing glass. The display tube operates under a high voltage (for example 25 kV) and generates X-radiation as a result of the electron bombardment on the glass and on the shadow mask in front of the screen.
Glasses for screens of display tubes must meet, inter alia, the following requirements: a high x-ray absorption, no discoloration as a result of electron radiation and x-radiation and a satisfactory meltability and processability. Said glasses preferably contain no or little PbO in order to suppress discoloration as a result of electron radiation. To obtain the desired x-ray absorption, said screen glass comprises 5-13 wt % BaO and 5-13 wt % SrO and, optionally, several percent ZrO.sub.2.
The cone glass must meet, inter alia, the following requirements: a high x-ray absorption, a high electrical resistivity and a satisfactory meltability and processability. Discoloration as a result of electron radiation is to be minimized. Since the wall thickness of the cone is smaller than the wall thickness of the screen, the x-ray absorption coefficient .mu. (expressed in cm.sup.-1 at a wavelength of 0.6 .ANG.) of cone glass must be higher than that of screen glass. In the current cone glass, .mu. has a value.gtoreq.65 cm.sup.-1 (at 0.6 .ANG.). This high .mu.-value is attained by a high concentration of PbO in the cone glass. At equal concentrations of PbO and SrO, the x-ray absorption effectiveness of PbO is 1.5 times that of SrO. In a typical example, the current cone glass comprises approximately 23 wt % PbO. Since the raw material is very expensive, the current cone glass comprises very little SrO and the sum of BaO+SrO is approximately 1.5 wt %.
As the wall thickness of the neck is thinner than that of the cone, neck glass should have an even higher .mu. value, namely .mu..gtoreq.100 cm.sup.-1 (at 0.6 .ANG.). This is attained by means of an even higher concentration of PbO, namely 30-35 wt %. The neck is manufactured from drawn glass tubing.
Future government measures will compel manufacturers of TV receivers and monitors to take back and recycle devices at the end of their lifetime. In this manner, the amount of waste and hence the space occupied by it at rubbish dumps is reduced. In addition, raw materials and energy can be saved. The glass display tube of the TV receivers to be recycled is the component with the highest weight, namely up to approximately 20 kg. It appears from the foregoing that the components of the display tube have different glass compositions. This complicates the recycling process of the glass display tube. For example, used cone glass, which comprises PbO, cannot be used to prepare new screen glass, because PbO causes brown coloration of the display screen during operation of the display tube. For the same reason a mixture of screen glass and cone glass cannot be used to prepare new screen glass. A possible solution would be to separate the screen from the cone, for example, by means of a diamond grinding wheel. After a cleaning treatment, the screen can be recycled to form new screen glass and the cone can be recycled to form new cone glass. However, this method is very laborious.
In European Patent Application EP-A-525226, a description is given of a method of recycling display tubes. A complete display tube is crushed in a crusher and ground into a mixture of glass fragments. Metal parts of the display tube, such as the shadow mask, are removed. The glass fragments are cleaned with water to remove lacquer, phosphors etc. The screen glass and cone glass fragments are sorted out manually, whereafter the sorted types of glass are separately reused. Said document states that up to 10% of used glass fragments are added to the glass melt. A disadvantage of the known method is the time-consuming manual sorting of screen glass and cone glass.